One of the most delicate of all surgeries is that in which the skull is opened and the brain is penetrated to reach a point of interest, such as an embedded brain tumor which the surgeon must remove. The surgeon first must determine the location of the tumor, and then determine the optimum point of entry through the skull and into the brain to reach the tumor, taking into consideration the anatomy and physiology of the portions of the brain that will be penetrated, and possible movement of the brain and tumor within the skull resulting from loss of fluid and pressure when the skull is opened. Having selected the point of entry, the surgeon then must select and maintain the proper angle of penetration so that the surgical tools will reach the tumor and be able to remove it.
Recent advances in imaging technologies including magnetic resonance imaging (MRI), computer tomography (CT), and positron emission tomography (PET) have greatly improved the ability to determine non-invasively the structure within almost any portion of the human body. This ability is especially valuable in diagnostics. Such imaging technologies permit the determination of the exact location of a point of interest such as a tumor within a region of the anatomy such as the brain without any preliminary exploratory surgery. Scans of the brain are taken along a series of planes, the data is digitized, and the digitized data can be converted by computer into three-dimensional images showing the size and location of the tumor within the brain. The surgeon then uses this computerized information with other medical knowledge to determine the optimum point of entry into the brain. The computerized information is available to the surgeon during surgery to help the surgeon determine the depth of penetration required to reach the tumor and to estimate the amount of material that must be removed. This diagnostic data cannot, however, monitor changes in position of the brain and tumor within the skull during surgery resulting from loss of fluid and pressure when the skull is opened.
One of the most difficult aspects of brain surgery is estimating and maintaining the slope of entry along which the surgeon should guide the tools through the pre-determined point of entry to the point of interest within the brain. Even when the surgeon has computerized scans showing three dimensional images of the exact pre-surgery location of the point to be reached, determining and maintaining the slope of entry necessary to reach that point inside the brain from the predetermined point of entry is, at best, sophisticated guesswork. Simple geometry dictates that, even when the exact point of entry into the skull is preselected, there are an infinite number of slopes of entry through that point into the brain. The surgeon can only estimate the correct slope of entry and depth of penetration, based on knowledge and years of experience. FIG. 1 illustrates what can happen when a surgeon errs even slightly in estimating or maintaining the slope of entry. As may be seen, the surgeon, even though starting precisely at the preselected point of entry "P" and directing the surgical tools as best as can be determined in the direction of the tumor "T", nevertheless has missed a substantial portion of the tumor to be removed. The surgeon must reposition the penetrating tools through the brain in order to remove the remaining part of the tumor. In addition, except by comparing the amount of material removed with the mass of material apparent from the computerized image, the surgeon cannot determine if all the tumor has been removed.
Accordingly, there is a need for a method and apparatus that will assist in the guidance of a penetrating tool into a solid three-dimensional object to reach a point of interest therein. In particular, there is a need for a method and apparatus that will assist a brain surgeon in guiding a penetrating tool into a patient's brain to reach a point of interest, such as a brain tumor that has previously been identified and located by MRI or other diagnostic techniques.
The following description of the invention throughout this patent specification will be given in terms of a method and apparatus for assisting a surgeon in guiding a penetrating tool to a brain tumor, the location of which has been previously determined by MRI diagnostic techniques, and for monitoring movement of the tumor during the procedure. It will be recognized, however, that the basic principles of the invention are not so limited, and can be applied to any problem dealing with determining and maintaining the slope of entry into a solid three-dimensional object to reach a pre-determined point of interest therein.